Automatic by-pass for repeaters



ug. 9, 1949. R. H. HERRICK AUTOMATIC BY-PASS FOR REPEATERS Filed April 17,` 1947 5 Sheets-Sheet 1 k .mvv RUN mm E. um Nd IW s O R R. H. HERRICK AUTOMATIC BY-PASS FOR REPEATERS Aug. 9, 1949.

3 Sheets-Sheet 2 Filed April 17, 1947 mmm @NNN

wl/ENTOR Roswell H. Herrick 5 Sheets-Sheet 5 Filed April 1'7. 1947 mom nm om JNVENTOR RosweH H. Herrick IM Aftornevs Patented Aug. 9, 1949 UNITED STATES TENT OFFICE AUTOMATIC BE7-PASS FOR REPEATERS Application April 17, 1947, Serial No. 742,037

2l Claims. (Cl. 17g-44.)

Fihe present invention relates to signal current transmission systems and has for one of its obu jects the vprovision of improved apparatus for ley-passing a signal current repeater when the repeater fails or is otherwise rendered inactive. This application is a continuation-impart of copending application Serial No. 478,99i, iled March 5, 1943, now abandoned,

It is another object of the invention to provide an improved repeater by-pass arrangement, wherein the facilities used to render the Icy-pass channel active when the signal current repeater becomes inactive are wholly static in character and include no moving parts.

According to another and more specific object of the invention, a bias controlled non-linear iinpedance element, such, for example, as a copper oxide rectifier, is provided in the by-pass chantnel, whicl element is normally biased under the control of an amplifier included in the repeater to block the by-pass channel against signal current transmission therethrough.

In accordance with still another object of the invention, one of the stages of the amplifier used repeater is provided with a biasing elewhich is utilized to supply the channel blocking bias across the bias controlled impedance element of the l.oy-pass channel.

Yaccording to a further object of the invention, the impedance of a balancing network in eluded in the repeater is automatically changed each time the bi1-pass channel is rendered ac" tive or inactive.

it is further object of the invention to provide an improved Aand exceedingly simple are rangement for controlling the impedance of a balar network of the chaiacter utilised in the system of a signal current repeater.

In .cool ance with a still fur ier object of the ihven a bias controlled impedance eleoi the Sta-tic type is utilized in the baiig network to control the impedance oi the network.

The invention, both as to its organization and method of operation, together with further objects and aavantages thereof, will best be understood reierence ,to the speciiication taken in connection with the accompanying drawings, in

2 3 illustrate signal current ssion systems provided with. different emb d ments of the present invention. In the drawings, reference numerals diiering in the hundreds digits, but having the same tens rnd u digits, are used to identify corresponding elements oi the three illustrated systems,

Referring now to the drawings, and more particularly to Fig. 1 thereof, the signal current transmission system there illustrated is adapted to be connected between a pair of two conductor lines itl and EQ2. Briefly considered, the system comprises a signal current repeater having a channel 93 for transmitting signal currents from the line lill to the line |02, and a second channel its for transmitting signal currents from the line 'm2 to the line lill, The system also includes a 1cy-pass channel I2@ for transmitting signal currents between the two enumerated lines in either direction when either of the two channels of the repeater fails or is otherwise rendered inactive. More speciiically, the repeater comprises the usual hybrid coils m5 and IUS, the line windings of which are coupled to the conductors of the lines itil and M732, respectively, through direct current blocking condensers ci large capacity. Thus the two line windings 5,151) and liliic of the hybrid coil It are shown as being coupled to the conductors of the line ll through the direct current blocking condensers llb and Illia. Similarly, the line windings lilc and iilib of the hybrid coil les are illustrated as being coupled to the conductors of the line lili! through the direct current blocking condensers l lea and l Mib. The hybrid system provided at each end of the repeater is also equipped with a balancing network having the function of balancing the impedance of the associated line to minimize the coupling between the input and output sides of the hybrid system and thus reduce the tendency of the repeater to sing. Thus the inner terminals of the line vwindings Esiib and liic of the hybrid coil |05 are connected tou gether through a balancing network lul which serially includes an adjustable resistor lille, a copper oxide rectiiier Mild, and the parallel connected winding of aniron core choke liilb and an adjustable condenser l sla. In a similar manner, the inner terminals of the hybrid coil windings illic and luth are connectedtogetherthrough a balancing network lull which comprises an adjustable resistor i880, -a copper oxide rectiiier lg3d, and an adjustable condenser lili-3a shunted by the winding of an iron core choke H6817.

The signal current channel U23, provided to transmit signal currents from the line is! to the line lilZ, comprises a single stage of amplication which includes the three electrode electron discharge tube lille. This tube, which may be a conventional triode, is of the indirectly heated cathode type and is provid-ed with input electrodes mib and w3c which are coupled :to

the line windings of the hybrid system |05 by means of an input circuit comprising the winding |05c of the hybrid coil |05 and a cathode biasing resistor |||a shunted by a signal current by-pass condenser ||2a. The output electrodes |090 and |090 of this tube are included in an output circuit which comprises the cathode biasing element |||a, a source of anode potential 3 shunted by a signal current by-pass condenser H4, the left portion of the hybrid coil winding |050, the incircuited portion of the resistor |000, the copper oxide rectifier |0801, the winding of the choke coil |0812, and the left portion of the hybrid coil winding |000.

The channel |04 for transmitting signal currents from the line |02 t0 the line |0| similarly comprises a single stage of amplication wherein the three electrode vacuum tube ||0 is provided with input electrodes ||0b and ||0c coupled to the hybrid coil |06, and with output electrodes ||0a and H00 which are coupled to the hybrid coil |05. More specifically considered, this tube is provided with an input circuit which serially includes the signal coil |0Ba of the hybrid coil |06 and a cathode biasing resistor |||b shunted by a signal current by-pass condenser ||2b. The output circuit of the tube ||0 serially includes the cathode biasing element Illb, the space current path through the tube ||0, the right portion of the hybrid coil Winding |050, the incircuited portion of the resistor |010, the copper oxide rectifier |01d, the winding of the choke coil |0117, the right portion of the hybrid coil winding |0511, and the source of anode potential Briefly considered, the by-pass channel |20' comprises a coupling transformer |2| which is equipped with a secondary winding divided into four sections |2|b, |2|c, |2|d and |2|e, and is provided with a primary winding |2|a bridged across the conductors of the line 0|. This channel further comprises a coupling transformer |22 having a secondary winding divided into two sections |221) and |220 and is provided with a primary winding |220 bridged across the conductors of the line |02. The three secondary winding sections |2|'b, |2|0 and |221), in combination with a copper oxide rectifier |23, form a by-pass path, individual to the signal current channel signal currents in either direction between the two lines 0| and |02. Similarly, the three secondary winding sections |2|d, |2|e and |220, in combination with the coppergoxide rectier |24,

form a by-pass path. individual to the Signal current channel |04 of the repeater, for transmitting signal currents in either direction between the two enumerated lines. For the purpose of rendering these two paths active when the respective corresponding channels of the repeater if are rendered inactive, a source of direct current in the form of a battery |25 is provided for biasing the two rectiflers |23 and |24 in the proper direction to render these elements conductive.

In considering the normal operation of the |03 of the repeater, for transmitting 34 brid coil winding |061), the incircuited portion of the resistor |080, the rectier |0861, the winding of the choke coil |080, the left portion of the hybrid coil winding |060, the space current path between the anode |09a and the cathode |090 ofv the tube |09, and the cathode biasing resistor |||a to the grounded negative terminal of the source H3. through the cathode biasing resistor Illa, the

potential of the cathode |090 becomes positive with respect to the associated control grid |091). More speciiically, the bias voltage developed across the resistor |||a is suiiicient to bias the tube |00 to a, point midway along the linear portion of the operating characteristic of the tube. In a similar manner, when electron emission within the tube ||0 is initiated, space current flow through the output circuit of this tube is started, whereby a bias Voltage is developed across the cathode biasing resistor ||ib which is of suicient magnitude to maintain the correct potential diierence between the input electrodes I |00 and |00 of the tube ||0.

When current flow through the two copper oxide rectiers |0101 and |08d is established in the manner just explained, these two elements are eiectively biased to present a relatively low, resistive impedance to signal currents which may traverse the two balancing networks |01 and |08. In this regard it is noted that each of the four copper oxide rectiiiers |01d, |08d, |23 and |24, as provided in the system shown in Fig. 1 of the drawings, as well as the corresponding rectiiier elements of the modiiied systems shown in Figs. 2 and 3 of the drawings, eiectively constitutesanon-linear impedance andis capable of being biased by means of a direct voltage of the cor.. rect polarity and amplitude, to prevent signal current now therethrough. Thus each such ele- 49 ment effectively constitutes an iniinite impedance when properly biased. When any one of these elements is, on the other hand, biased with a direct voltage of the opposite polarity and correct magnitude the element will oier an innnite resistive impedance of relatively low value to signal current ow therethrough. With space current iiow through the two tubes |09 and ||0 established in the manner explained above, the two rectifier elements |08d and |0111 are effectively biased to present a low resistive impedance to signal current flow through the two balancing networks |01 and |08. Moreover, the extent of the Voltage drops across these two rectiiier elements is such that they will pass both the positive and negative halves of signal current waves entering the two balancing networks. In considering these two networks, it is noted that the circuit constants of each network are proportioned so that the overall impedance of each network eifectively matches, both in magnitude and phase angle, the impedance 0f the associated line as measured at the line terminals at the associated hybrid coil. Thus the series resistance through the rectifier element |01d and the incircuited portion of the resistor |010 is proportioned, through adjustment of the latter resistance element, substantially to equal the eiiective series resistance of the line |0| as measured at the line terminals of the two hybrid coil windings |051) and |050. In a similar manner, the effective reactive impedance represented by the parallel connected circuit elements |010 and |01b is proportioned substantially to balance the effective reactive impedance, including the combined capacitance of the two condensers H50 and H517,

As current now is thus establishedY as measured at. the liney terminals of the two hybrid coil windings |0511 and |05c. Ina similar manner, the constants of the circuit elements lila, |0811, |08c` and ltd included in the balancing network l are so proportioned that. the overall impedance of this network effectively balances the impedance of the line |02 as measured at the terminals of the hybrid coil windings |061) and Ic.

After space current iow through the two tubes tot and l, l@ is established in the manner just explained, the bias voltages developed across the cathode biasing resistors lila, and llib are respectively utilized to bias the two rectifier elements it and iZt in directions such that the two parallel sections of the, by-pass channel t are effectively rendered inactive, whereby signal current flow therethrough is blocked. |Ihus it will be noted that the voltage across the resistor illu is impressed acres; the rectifier |23 over a path which includes the battery i125 and the transformer secondary winding sections S2521, Sle and itil), This bias voltage, i e., that across the resistor ll la, is opposed by thev voltage of the battery E25 but is substantially greater than that of the battery,l such that a net voltage: equal to the difference between the two individual voltages is applied across the rectier element |23 in direction which enhances the non-conductivity oi this rectifier. More speciiically, thev net bias voltage serves to bias the rectifier element |23 substantially below the signal current cut-on? point thereof, thereby to block the upper intermediate section of the by-pass channel E20 against the transmission of signal currents between the two lines lill and |02. In a similar manner, the voltage across the cathode biasing l sistor l lib as opposed by the voltage of the battery l is applied across the rectier |24 over a path including the transformer winding ions iilid, ldlc and |22c to bias the rectifier ist substantially beyond the signal current cuto' point thereof, whereby the lower intermediate section of the bypass channel i2@ is blocked .nst the transmission of signal currents beeen the two lines it! and m2.

Vfith the signal current transmission system thus conditioned for operation, a signal voltage .1 ressed across the conductors of the line 80| uces a corresponding signal current flow rough the .line windings ist?) and iilc of the rid coil itt, the series connected elementsv of ively large and have the sole function of pre- Trenting any part of the space current traversing the tube l it from flowing over the conductors "Pla ne signal currents traversing the two :findings iiibb and lebe of the hybrid coil c a corresponding voltage to be induced f signal winding ittici', of the hybrid coil |05, w ce. voltage is impressed between the input el trodes its?) and idee of the amplifier tube to produce corresponding variations in the irrent traversing the output circuit of this tube. ampliiied signal current appearing on the output side of the tube ist divides between three Ael branches of the output circuit, one coming the path through the balancing network and the left portions of the hybrid coil winditb and Mite, a second comprising the D. C. cching condensers Mea and llb and the primary winding i252@ ofv the transformer |22, and

the third comprising the condensers Htc and Mtb, the conductors of the line |02 and the equipment terminating this line. at the distant end thereof. The signal currents traversing the two halvesof each of the hybrid coil windings |0512 and |060 are obviously of opposite polarity and, due to the action of the network |05 in balancing the external impedance as measured at the line terminals of these windings, are substantially equal in magnitude. Accordingly, the signal voltage induced in the signal winding la of the hybrid coil |06 is of a negligible value. 'Ihus the hybrid system operates to permit signal currents to be transmitted from the line lul through the channel |03 to the line |02, while at the same time preventing signal current energy from being passed to the input side or" the channel lud to produce an oscillatory condition of the repeater. In a manner clearly apparent from the above explanation, a signal voltage appearing between the conductors of the line 02 is transmitted to the input side of the channel |04 through the hybrid coil lut, amplified through the tube i lil and, in its amplified form, impressed between the conductors of the line |0|. In this case the ybrid system comprising the hybrid coil |05 and the balancing. network |01 functions to prevent any substantial portion of the signal current energy from being transmitted to the input side oi' the ampliiier tube |09 in the channel |03.

In the event either of the two signal transmitting channels |03 and its is rendered inactive, due to a failure of an amplier tube included therein, the corresponding section of the by-pass channel |20 is immediately rendered operative to transmit signal currents between the two lines lill and idg in either direction. In this regard it will be understood that should the cathode heater supply source fail, for example, both arnpliler tubes 00 and l0 will immediately be rendered inactive, thereby to block the transmission or" signal currents through the channels |03 and its. On the other hand, should only the amplifier tube ist become defective, by reason of the cathode heater thereof burning out, for example, only the signal current channel |03 is rendered inactive. More generally considered, if for any reason space current flow through the amplifier tube lt is interrupted the voltage across the cathode biasing resistor lilo is immediately reduced to a negligible or Zero value. When this voltage disappears, the voltage of the battery |25 is impressed across the rectifter element |23 over a path which includes the resistor la and the secondary winding sections lZlb, |2|c and i221), in the proper direction to render the rectiier element |23 conductive. The extent of this bias voltage as derived from the battery |25 is, in the absence oi' the opposing voltage normally present across the resistor lllc, suliicient to bias the rectifier element |23 midway along the linear portion of its operating characteristic. With the rectiiier element |23 biased in this manner the upper intermediate section of the by-pass channel eiectively comprises a closed loop having serially included therein the three secondary winding sections |2ib, |2|c and |2211, the rectier 23, the resistor llia and the battery |25. Thus signal voltages appearing across either of the two lines tu! and |02 may be transmitted through the by-pass channel |20 to the other v:liow through the line windings and the balancing network |08.

envases .through the' tube |09 and the rectifier element `|0811 is arrested, the resistive impedance of this rectifier element is effectively increased to an infinite value, thereby to prevent signal current |05?) and |066 By controlling the rectifier element |0811 in this manner to increase the shunt resistance across the conductors of the line |02 to substantially an infinite value, the signal current losses in the hybrid system including the balancing network |08 are materially reduced to permit a larger portion of the signal current voltage appearing across either of the two lines and |02 to be transmitted to the other line.

It will be understood from the above explanation that if for any reason space current iiow through the amplifier tube ||0 is interrupted, such that the voltage across the biasing resistor |||b is reduced to Zero, the battery |25 operates to bias the rectifier element |24 in the proper clirection to render the lower intermediate section of the by-pass channel |20 operative to transmit signal currents in either direction between the two lines |0| and |02. Concurrently therewith the resistive impedance of the rectier element |0111 is increased substantially to infinity to reduce the signal current losses through the hybrid system comprising the balancing network 0l and the hybrid coil |05.

4From the foregoing explanation it will be understood that when both of the two channels |03 and |04 are rendered inactive, the two parallel intermediate sections of the by-pass channel |20 are automatically rendered active. Concurrently therewith the signal current losses through both hybrid systems are reduced to an exceedingly low value by increasing the resistive impedance of the two rectifier elements |0`|d and |08d. On the other hand, when only one of the two channels |03 and |04 is rendered inactive, the correspondingintermediate section of the bypass channel is rendered active and the opposite signal current channel is rendered inactive. Thus when the channel |03 is rendered inactive the rectifier element |08d operates effectively to open the signal current path through the line windings |0619 and |000 of the hybrid coil |06. Since signal currents cannot transverse these two windings with the rectifier element |08d biased to block such signal current flow, obviously no signal voltages may be induced in the winding |0611 for amplification through the channel amplifier tube ||0. In a similar manner, when the channel |04 is alone rendered inactive, the rectifier element |0'ld effectively opens the signal current path through the line windings |0519 and |050 to prevent signal voltages appearing across the line |0| from being transmitted by induction to the winding |05a for amplification through the channel amplifier tube |09. It will be understood therefore that when either of the two channels |03 and |04 is rendered inactive, signal current transmission between the two lines |0| and |02 may only be effected through the bypass channel |20.

In the modified system arrangement shown in Fig. 2 of the drawings a by-pass channel 220 provided with a single intermediate section is used. This section of the by-pass channel serially includes the copper oxide rectifier element 223 which is normally biased to block the channel against signal current transmission by the voltage appearing across the cathode biasing resistor 2| The indicated biasing resistor is com- 8 mon to the output circuits of the two amplifier tubes 209 and 2|0. More specifically considered, the two vacuum tubes 209 and 2 |0 are of the iilamentary type and are connected to be energized in parallel by a battery 221. A center tapped resistor 228 is connected in parallel with the two cathodes 209e and 2 |0c to carry equal portions of the space current traversing each of the two tubes, and thus provide a vehicle whereby the voltage across the resistor 2|| is positively applied to both terminals of each cathode. The arrangement of the hybrid systems as provided in the Fig. 2 embodiment of the invention, also differs from that shown in Fig. 1 of the drawings in that the signal winding 205a of the hybrid coil 205 is included in the output circuit of the amplifier tube 2|0 and the signal winding 206a of the hybrid coil 206 is included in the output circuit of the other amplifier tube 209. The signal voltage appearing across the conductors of the line 20| is impressed between the input electrodes of the amplifier tube 209 through a transformer 230 having primary windings 2301) and 230e, which are bridged across the center taps of the hybrid coil line windings 205D and 2050 in series with a rectifier 240 and a large condenser 229 of low impedance to signal currents within the voice frequency range. The secondary winding 230e of the transformer 230 is connected between the input electrodes of the amplifier tube 209 in series with the cathode biasing resistor 2||. In a similar manner signal voltages appearing across the conductors of the line 202 are impressed between the input electrodes of the amplifier tube 2|0 through a transformer 226 which is provided with two primary windings 22617 and 226e. These two windings are bridged across the tapped center points of the hybrid coil line windings 206b and 206e in series with a rectifier 24| and the condenser 229. The transformer 226 is also equipped with a secondary winding 226e which is connected between the input electrodes of the amplifier tube 2|0 in series with the cathode biasing resistor 2||. In the Fig. 2 arrangement the net bias voltage, i. e., that represented by the difference between the voltage across the cathode biasing resistor 2|| and that across the battery 225, is normally utilized not only to bias the rectifier element 223 beyond cut-off, but also to bias the four rectifier elements 201d, 208d, 240 and 24| to a relatively low resistive impedance. Thus the difference voltage between the voltage across the battery 225 is applied across the terminals of the rectifier elements 20`|d and 240 over a path which includes the two windings 2301) and 230e, the right portions of the hybrid coil line windings 205b and 205e, the incircuited portion of the resistor 201e and the winding of the choke coil 201D. This difference voltage is of the correct polarity to cause current to traverse the rectifier elements 20'id and 240 and is of a magnitude such that the resistive impedance of these elements to signal current flow is relatively low. The same diiference voltage is applied across the terminals of the rectifier elements 208:5 and 24| over a path which includes the two windings 2201) and 225e, the left portions of the hybrid coil windings 203D and 206e,

the incircuited portion of the resistor 2001:, and the winding of the choke coil 2081 Here again, the voltage is applied across the rectifier elements 208d and 24| in a direction such that current traverses these elements and is of a magnitude sufficient to render these elements of relatively low resistive impedance to signal current now therethrough.

In general, the mode of operation of the Fig. 2 arrangement resembles that of the System shown in Fig. 1 of the drawings. Thus signal voltages appearing across the conductors of the line Eel and impressed across the primary windings i3d?) and 23de of the transformer 23B through the line windings of the hybrid coil 205 are amplied by the tube 2&9 and impressed upon the conductors of the line 2M through the windings or the hybrid coil 2%. Similarly, signal voltages appearing between the conductors of the line Zo and impressed across the primary windings 22th and 226e of the transformer 22% through the line windings of the hybrid coil 2%;

are amplied by the tube El@ and transmitted through the hybrid coil Zo to the line l. Normally, the by-pass channel 22] is rendered inactive due to the fact tiat the rectifier element 22S is biased by the voltage across the resistor iii to prevent signal currents from traversing the intermediate section of this channel. Should, however, space current flow through either or both oi the two tubes Zlii and 2li) be interrupted, the voltage across the biasing resistor 2li is either halved or reduced subtsantially to Zero. In either case the voltage across the resistor 2li is decreased to a Value lower than that of the battery 225, whereby the polarities of the bias voltages impressed across the five rectifier elements Elib, 2M, Zbld, 223 and Zlid are reversed. As a consequence, the rectifier element 223 is rendered conductive to unblock the intermediate section of the by-pass channel 229 and the resistive impedance of each of the two rectier elements Zllid and 28d is increased to substantially an infinite value, thereby to block the balancing networks 2M and Ella against signal current transmission therethrough. After the bypass channel 22u has been activated in the manner just explained, it is rendered operative to transmit signal currents between the two lines and mi2 in either direction. Rectiers 213B and are also rendered substantially nonconductive thereby effectively disconnecting the primary windings of transformers 235i and 226 from the hybrid transformers 205 and 26B, respectively. With this arrangement, if either of the two signal current channels 2% and 2M is rendered inactive, both channels are in effect disconnected from the hybrid transformers to prevent singing of the amplier which remains active. Both of the line balancing networks are in'efect disconnected from their associated hybrid transformers to eliminate losses which would otherwise occur therein, thus increasing the ecacy of the bypass channel.

The modified embodiment of the invention shown in Fig. 3 of the drawings is substantially similar to the arrangement illustrated in Fig. 2 except that a direct current bridge comprising the three resistors 33l, 332 and 334, and the space current path through the added control tube 33t is utilized for the purpose of biasing the ive rectier elements 340, 34|, 30M, 3ll8d and 323 in accordance with the voltage across the cathode biasing resistor 3H. The control tube 333 is oi the well known triode type -employing a lamentary type cathode v33de which is energized from a direct current source 339 and is shunted by a bias voltage equalizing resistor 335. The voltage developed across the cathode biasing resistor 3l l as a result of space current flow through the two tubes39 and 3l is positively applied to the control grid 3331) of the control tube 333 through a resistor 336 which is shunted by a lter condenser 331. This bias voltage, which normally prevails when the two tubes 3F59 and Sli? are operating, is opposed by the voltage of a battery 333, which opposing voltage is of lesser magnitude than that normally appearing across the resistor Hi Thus under normal conditions, the tube 333 operates with its control grid 33311 biased positively with respect to the cathode 333C. The relative magnitudes of the two opposing bias voltages are so proportioned that under normal conditions the rene-tance ci the space current path through the tube i. e., that between the terminals B D of the bridge circuit, is equal to the resistce of the resistor 334. The resistors 33| and on the other hand, have equal resistance values. This value is substantially twice that of the resistance ci' the resistor 33d and twice that or" the normal resistance of the space current path through the tube 333.

With the bridge circuit arranged in this manner, and the resistance values of the four arms thereof thus established, current normally traverses the space current path of the control tube 33S over a circuit which includes the source of anode potential M3 and the high Value resistor The two resistors 33! and 33d are energized in series directly from the source M3. Since the current flow through the resistor 332 is equal to that through the resistor 331i and the latter resistor is possessed of a resistance value only half that of the resistor 332, the voltage drop across the resistor 332-is twice that across the resistor 33d, whereby the potential at the terminal A of the bridge circuit is normally positive with respect to the terminal D of this circuit. The potential difference between the two points A and D is of suflcient magnitude to block the rectifier element 323 against signal current transmission. From an inspection oi the circuit connections, it will be apparent that the voltage between the points A and D of the bridge circuit is applied across the terminals of the rectifier element 323 over a ypath which includes the ywinding 32217 and the winding sections 32Ib and 32ic in the proper direction to maintain the rectier element 323 non-conductive. This voltage is also applied lacross the terminals of the four rectier elements 3,40, 3M, 30M and Siiild in the proper directions to bias these elements to a relatively low value of resistive impedance. The circuits over-which the voltage between the terminals A and D of the bridge circuit is impressed across vthe four rectifer elements 34B, 3M, Sild and Seed will be clearly apparent from the above explanation with reference to the arrangement shown in Fig. ,2 of the drawings.

From the above .explanation it will be understood that under normal operating conditions when both channels 3&3 and @be of the repeater are operating, the rectier element 323 is biased to block the intermediate section of the ley-pass channel 320 against signal cur-rent transmission therethrough, andthe four rectifier elements Stil, Sli'lldandtd are biased to present relatively low resistive impedancesto the passage of signal currents therethrough. `Should either or both of the two tubes 3ds and 3m fail, however, the voltage drop across the cathode biasing resistor lill is either halved or decreased to zero. In either case, the voltage of the battery i338 predominates over the voltage across the resistor 3H., thereby to increase negatively the bias on the control grid l1 33317 of the tube 333. The resulting increase in the resistance of the space current path through the tube 330 serves to decrease the current flow through the resistor 332, whereby the potential of the bridge circuit terminal D is increased to a value greater than the value of the potential existing at the point A of the bridge circuit. In other words, the point D becomes positive with respect to the point A, such that the polarities of the bias voltages impressed across the ve rectier elements 340, 34|, 30'ld, 3080i and 323 are reversed. As a consequence, the by-pass channel 320 is rendered operative to transmit signal current between the two lines 30! and 302 in either direction and the resistive impedance of the two rectifier elements 301:1 and 30811 is increased substantially to infinity, thereby to reduce the signal current losses through the two balancing networks 301 and 308. Rectiiiers 340 and 3M are also rendered substantially non-conductive thereby effectively disconnecting the primary windings of transformers 325 and 326 from the hybrid transformers 305 and 306 respectively. It will be apparent from the above explanation that should only one of the two amplier tubes 309 and 3I0 fail, with a resulting activation of the bypass channel 320, both of the amplifiers are in effect disconnected from the hybrid transformers to prevent singing of the amplier which remains active. Only the by-pass channel 320 is available for transmitting signal currents between the two lines in either direction. It will also be apparent that should the control tube 333 fail, such that the resistance of the space current path therethrough is increased substantially to ini nity, the by-pass channel 320 is unblocked and the four rectifier elements 340, 34|, 30ld and 300d are biased to the point of maximum resistive impedance. In order to preclude this possibility, the control tube 333 should be operated well below its operating limits as regards the voltage applied to its anode, the magnitude of space current flow through the tube, and the voltage impressed across the cathode 333e thereof. If such precautions are taken, the control tube 333 may be expected to exceed in life the tubes 309 and 3 l0 included in the signal current transmission channels of the repeater.

While three embodiments of the invention have been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention.

What is claimed is:

1. In a signal current transmission system, an ampliiier comprising an electron discharge tube provided with input and output electrodes, a biasing element traversed by the space current iiowing between said output electrodes and connected to provide an operating bias between said input electrodes, a by-pass channel paralleling said amplifier, a control device included in said by-pass channel and having a signal current cut-off point, means connecting said biasing element to said control device so that said control device is biased beyond said cut-OIT point when said amplifier is operating, thereby to block said by-pass channel against the transmission of signal currents therethrough, and a voltage source connected in opposition with the voltage across said biasing element, whereby the polarity of the bias on said device is reversed to render said by-pass channel active when said ampliiier is rendered inactive.

2. In a signal current transmission system, ampliers for transmitting signal currents in opposite directions and each comprising an electron discharge tube provided with input electrodes and an output circuit, a biasing element commonly included in said output circuits, means connecting said biasing element to provide an operating bias between the input electrodes of each tube, a by-pass channel paralleling said amplifiers, and means controlled by the voltage across said biasing element for at least partially blocking said by-pass channel against the transmission of signal currents therethrough.

3. In a signal current transmission system ampliers for transmitting signal currents in opposite directions and each comprising an electron discharge tube provided with input electrodes and an output circuit, a biasing element commonly included in said output circuits, means connecting said biasing element to provide an operating bias between the input electrodes of each tube, a by-pass channel paralleling said amplifiers, a control device included in said by-pass channel and having a signal current cut-oir point, and means connecting said biasing element to said control device so that said control device is biased beyond cut-01T when said ampliers are operating, thereby to block said by-pass channel against the transmission of signal currents therethrough.

4. In a signal current transmission system, amplifiers for transmitting signal currents in opposite directions and each comprising an electron discharge tube provided with input electrodes and an output circuit, a biasing element commonly included in said output circuits, means connecting said biasing element to provide an operating bias between the input electrodes of each tube, a by-pass channel paralleling said amplifiers, a control device of the static type included in said by-pass channel and having a signal current cutoi point, means connecting said biasing element to said control device so that said control device is biased beyond cut-off when said ampliers are operating, thereby to block said by-pass channel againstl the transmission of signal currents therethrough, and a voltage source connected in opposition with the voltage across said biasing element, whereby the polarity of the bias on said device is reversed to render said by-pass channel active when either one of said ampliiiers is rendered inactive.

5. In combination with a pair o1 amplifiers which are connected to a hybrid system provided with a balancing network, a normally inactive by-pass channel paralleling said ampliiiers and said hybrid system, means for automatically rendering said channel active, and means for automatically increasing the impedance of said netzvork when said by-pass channel is rendered acive.

6. In comblnation with a plurality of ampliiiers which are connected to a hybrid system provided with a balancing network, a by-pass channel paralleling said amplifiers and said hybrid system, means for automatically rendering said channel active each time one of said ampliers is rendered inactive, and means controlled by said one amplifier for automatically increasing the impedance of said network each time said channel is rendered active.

7. In combination with a pair of ampliiers which are connected to a hybrid system provided with a balancing network, a non-linear impedance element included in said network, a normally inactive by-pass channel paralleling said amplifiers and said hybrid system, means for automatically rendering said channel active, and

13 means for automatically controlling said nonlinear impedance element to increase the impedance of said network when said channel is rendered active.

8. In combination with a pair of ampliiiers which are connected to a hybrid system provided with a balancing network, a non-linear impedance element included in said network, a ley-pass channel paralleling said amplifiers and said hybrid system, means `ior automatically rendering said channel active each time one of said ampliers is rendered inactive, and means controlled by said one amplifier for automatically controlling said impedance element to increase the impedance of said network each time said channel is rendered active.

9. In combination with a signal current channel which is divided into two sections, a pair of parallel signal current paths, a pair of hybrid systems connecting each of said parallel signal current paths to two sections of said channel, a balancing network for each of said hybrid systems, a normally inactive by-pass signal current path connecting said two sections of said channel, means for automatically rendering said bypass signal current path active, and means for automatically increasing the impedance of said network when said by-pass signal current path is rendered active.

l0. In a signal current transmission system, a pair of amplifiers, a hybrid system coupled to said amplifiers and provided with a balancing network, a normally inactive by-pass channel paraileling said ampliers and said hybrid system, means controlled by one of said ampliers for rendering said by-pass channel active, and means controlled by said one amplier for automatically increasing the impedance of said network each time said by-pass channel is rendered active.

l1. In a signal current; transmission system, a p-air of amplifiers, each of said ampliers comprising an electron discharge tube provided with an output circuit, a hybrid system coupled to said amplifiers and provided with a balancing network, a normally inactive by-pass channel paralleling said amplifiers and said hybrid system, means for rendering said by-pass channel active in response to a termination of the current iiow in the output circuit of one of said amplifiers, and means controlled by said one ampliiier for automatically increasing the impedance of said network each time said channel is rendered active.

12. In a signal current transmission system, a pair of amplifiers, each of said ampliiiers comprising an electron discharge tube provided with input electrodes and an output circuit, a biasing element included in said output circuits, means connecting said biasing element to provide an operating bias between the input electrodes of said tubes, a hybrid system coupled to said arnpliers and provided with a balancing network, and means responsive to the reduction to substantially Zero of the Voltage across said biasing element for increasing the impedance of said network.

13. In a signal current transmission system, a pair of ampliiiers, each of said ampliers comprising an electron discharge tube provided with input electrodes and an output circuit, a biasing element included in said output circuits, means connecting said biasing element to provide an operating bias between the input electrodes of said tubes, a hybrid system coupled to said ampliners and provided with a balancing network, a by-pass channel paralleling said ampliers and said hybrid system, means controlled by the voltage across said biasing element for rendering said by-pass channel inactive, means also controlled by the voltage across said biasing element for normally establishing the impedance of said network at a relatively low value, and means for controliing said two last-named means to render said by-pass channel active and to increase the impedance of said network when the voltage across said biasing element is reduced substantially to` zero.

le. In a signal current transmission system, a pair of ampliers, each of said ampliers comprising .an electron discharge tube provided with input electrodes and an output circuit, a biasing element included in said output circuits, means connectinGr said biasing element to provide an operating bias between the input electrodes of said tubes, a hybrid system coupled to said ampliers and provided with a balancing network, a non-linear impedance element included in said network, a by-pass channel paralleling said ampli-ners and said hybrid system, a -second nonlinear inipedance element included in said channel, circuit means for applying the voltage appearing across said biasing element to said impedance elements in directions such that said bypass channel is rendered inactive and the impedance of said network is established at a relatively low value, and means for controlling said impedance elements to render said by-pass channel active and to increase the impedance of said network when the voltage across said biasing element is reduced substantially to zero.

15. In combination with a pair of amplifiers which are coupled to a hybrid system provided with a line balancing network, a common control tube coupled to said ampliners, a bridge circuit including in one of its arms the space current path oi said tube, whereby the resistive balance of said bridge is determined by the resistance of said space current path, and means controlled in accordance with the balanced condition of said bridge circuit for decreasing the impedance of said network and controlled in accordance with the unbalanced condition of said bridge circuit ior increasing the impedance of said network.

16. In combination with a pair of amplifiers which are coupled to a hybrid system provided with a line balancing network, a biasing element which is common to the input and output circuits of one stage of said ampliers, a common control tube, means for controlling the resistance of the space current path through said tube in accordance with the voltage across said biasing element, a common bridge circuit including in one of its arms the space current path of said tube, whereby the resistive balance of said bridge is determined by the resistance of said space current path, and means controlled in accordance with the balanced condition of said bridge circuit for decreasing the impedance of said network and controlled in accordance with the unbalanced condition of said bridge circuit for increasing the impedance of said network.

17. In combination with a pair of amplifiers which are coupled to a hybrid system provided with a balancing network, a biasing element which is common to the input and output circuits of one stage of each of said ampliers, a by-pass channel paralleling said ampliers and said hybrid system, means controlled in accordance with the voltage across said biasing element for selectively rendering said by-pass channel active and inactive, a control tube, means for controlling the resistance of the space current path through said tube, in accordance with the voltage across said biasing element, a bridge circuit including in one of its arms the space current path of said tube, whereby the balance of said bridge is determined by the resistance of said space current path, and means controlled in accordance with the balanced condition of said bridge circuit for decreasing the impedance of said network and for selectively rendering said Dy-pass channel inactive, and controlled in accordance with the unbalanced condition of said bridge circuit for increasing the impedance of said network and for selectively rendering said lay-pass channel active.

18. In a signal current repeater, a hybrid sys tem including a balancing network, a bias controlled rectier included in said network, and means for impressing a variable bias on said rectiiier, thereby to control the impedance of said network.

19. In a signal current repeater, a hybrid system including a balancing network, and means comprising a bias controlled rectiiier element included in said network for changing the impedance of said network.

20. In a signal current repeater, a hybrid sys,-V

ROSWELL H. HERRICK.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,034,703 Metzger Mar. 24, 1936 2,212,832 Holzler Aug. 27, 1940 2,229,089 Kinsburg Jan. 21, 1941 2,229,090 Kinzer Jan. 21, 1941 2,229,108 Maggio et a1. Jan. 21, 1941 

